The Possibility of Life Elsewhere in the Universe

Christopher F. Chyba

Department of Astrophysical Sciences

Princeton University


One characteristic of the scientific investigations undertaken for the International Geophysical Year (IGY) in 1957–1958 was an emphasis on global measurements. Studies of Earth’s ionosphere, for example—crucial for the theory of short-wave radio communication—required data from all over the globe. Coordinated international studies certainly long predated the IGY, but the use of globalized data collection to support improvements in a world-spanning communications technology was a harbinger of today’s “globalization,” a term that is now, 50 years later, nearly a cliché.

But the IGY required an even larger context. Upper atmospheric studies also needed an understanding of the interaction of Earth with the Sun. Understanding Earth required placing Earth in its solar system context. In this sense the IGY can also be seen as a harbinger of what is now called “astrobiology.” Writing in 1974, shortly after the Moon landings, Carl Sagan asserted that we could, for the first time, try to understand life on Earth in its cosmic context. Space travel revealed that this was not just a metaphor, but literally true: we could only hope to understand the origin and evolution of life on Earth by placing Earth in the context of its solar system and galactic environments. Moreover, our understanding of the prospects for life elsewhere is in turn strongly shaped by our expanding knowledge of life on Earth.

THE LIFE WE KNOW

A discussion of life elsewhere therefore naturally begins with a review of some key aspects of the biosphere here on Earth. On Earth’s surface, there is something like a thousand trillion kilograms of carbon locked up in the living things that we see easily with our naked eye—plants, animals, and fungi. Most of this “biomass” is in trees. But in the past couple of decades, we’ve also learned that there seems to be a similar biomass of microscopic organisms living in the oceans, and another comparable biomass—this learned from deep-Earth drilling projects—of microscopic organisms living underground, down to depths of at least several kilometers. It appears that at least a small fraction of this subsurface biosphere is independent of surface conditions—that is, there are microorganisms living underground today that would likely continue to thrive even if the Sun were to go out, and photosynthesis shut down, tomorrow. This is not true for a great deal of subsurface life, much of which directly or indirectly depends on the energy harvested from sunlight at Earth’s surface—e.g., because it depends on the organic molecules produced by photosynthesis, or depends on the oxidized molecules resulting from the oxygen liberated by photosynthesis. But it appears that some microorganisms—such as those that make their living by combining hydrogen (produced from subsurface water weathering rocks) with carbon dioxide dissolved—might really represent ecosystems that are independent of the surface. As long as liquid water would persist in Earth’s interior—and this will be the case as long as there is enough internal geothermal heating to sustain some layer in Earth’s rocks where liquid water exists—it seems likely that there will be a subsurface biosphere.

The elucidation of Earth’s subsurface biosphere changes the way we think about the prospects for life



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The Possibility of Life Elsewhere in the Universe Christopher F. Chyba Department of Astrophysical Sciences Princeton Uniersity One characteristic of the scientific investiga - here on Earth. On Earth’s surface, there is something tions undertaken for the International Geophysical like a thousand trillion kilograms of carbon locked up Year (IGY ) in 1957–1958 was an emphasis on global in the living things that we see easily with our naked m easurements. Studies of Earth’s ionosphere, for eye—plants, animals, and fungi. Most of this “biomass” example—crucial for the theory of short-wave radio is in trees. But in the past couple of decades, we’ve also c ommunication—required data from all over the learned that there seems to be a similar biomass of mi- globe. Coordinated international studies certainly long croscopic organisms living in the oceans, and another predated the IGY, but the use of globalized data col- comparable biomass—this learned from deep-Earth lection to support improvements in a world-spanning drilling projects—of microscopic organisms living un- communications technology was a harbinger of today’s derground, down to depths of at least several kilometers. “globalization,” a term that is now, 50 years later, nearly It appears that at least a small fraction of this subsurface a cliché. biosphere is independent of surface conditions—that is, But the IGY required an even larger context. Upper there are microorganisms living underground today that atmospheric studies also needed an understanding of would likely continue to thrive even if the Sun were to the interaction of Earth with the Sun. Understanding go out, and photosynthesis shut down, tomorrow. This Earth required placing Earth in its solar system context. is not true for a great deal of subsurface life, much of In this sense the IGY can also be seen as a harbinger which directly or indirectly depends on the energy of what is now called “astrobiology.” Writing in 1974, harvested from sunlight at Earth’s surface—e.g., be- shortly after the Moon landings, Carl Sagan asserted cause it depends on the organic molecules produced by that we could, for the first time, try to understand life photosynthesis, or depends on the oxidized molecules on Earth in its cosmic context. Space travel revealed resulting from the oxygen liberated by photosynthesis. that this was not just a metaphor, but literally true: we But it appears that some microorganisms—such as could only hope to understand the origin and evolution those that make their living by combining hydrogen of life on Earth by placing Earth in the context of its (produced from subsurface water weathering rocks) solar system and galactic environments. Moreover, our with carbon dioxide dissolved—might really represent understanding of the prospects for life elsewhere is in ecosystems that are independent of the surface. As long turn strongly shaped by our expanding knowledge of as liquid water would persist in Earth’s interior—and life on Earth. this will be the case as long as there is enough internal geothermal heating to sustain some layer in Earth’s rocks where liquid water exists—it seems likely that THE LIFE WE KNOW there will be a subsurface biosphere. A discussion of life elsewhere therefore naturally begins The elucidation of Earth’s subsurface biosphere with a review of some key aspects of the biosphere changes the way we think about the prospects for life 

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 FORGING THE FUTURE OF SPACE SCIENCE elsewhere. If deep biospheres are possible, even in the investigations of the space between the stars, the so- face of harsh surface conditions, then the prospects for called interstellar medium (ISM). Probing the ISM subsurface life on Mars, Europa, or elsewhere seem at radio frequencies reveals that there is a rich carbon greater. But we must remember that the requirements chemistry throughout our galaxy; to date there are for habitability are not necessarily the same as the nearly a hundred carbon-based molecules observed in requirements for the origin of life. On Mars, it is at the ISM. There is no comparable suite of silicon-based least possible that life originated at the surface, where molecules seen. Now, the ISM was not investigated it could take advantage of the tremendous available primarily to test the hypothesis of silicon-based life. energy from the Sun, and then migrated to the subsur- Rather, scientists simply wanted to learn what was face as the surface became a freeze-dried desert. In the out there—this was largely exploratory science, not case of Jupiter’s moon Europa, which likely harbors a hypothesis-testing science. But as a result of explora- subsurface ocean of liquid water, it seems unlikely that tion, it seems more likely that carbon will be the basis there were hospitable surface conditions for more than for chemical life elsewhere in the universe, should any a fleeting moment, if that, early in solar system history. exist. Of course, this is at most an implication, not a For there to be life in Europa’s ocean, it would likely strong conclusion. have to have originated in the subsurface. We do not understand the origin of life well enough to assess the WHAT IS LIFE? plausibility of this scenario. In both of these cases—Mars and Europa—life All life we know on Earth is carbon-based, but it shares seems at least possible because of the likelihood of the many more commonalities as well. Its basic biochem- presence of subsurface liquid water. It is fair to ask: must life depend on liquid water? How many of the apparently universal characteristics of life on Earth are requirements for life everywhere? Life on Earth is carbon-based; is this a general requirement or simply one of many possible alternatives? Of course, we can not answer this question with confidence until we know more and have explored farther. But we are already getting some hints to the answer. Consider alternatives to carbon. Speculation has often focused on silicon-based life as an alterna- tive to the carbon-based life we know. The theoretical reason for this can be seen by glancing at the periodic table of elements; silicon sits directly beneath carbon in this table, which is a short-hand way of saying that its chemical properties are similar. Since silicon, like carbon, is also an abundant element in the universe, it might seem to provide a good alternative. But in fact, silicon’s chemistry is more limited; except under ex- traordinary laboratory conditions, silicon atoms will not form double bonds with themselves, as carbon atoms do, so silicon chemistry is substantially more restricted than carbon chemistry. This is a consequence of the fact that the silicon atoms are simply bigger than carbon atoms, making double bonds much more difficult. On top of this theoretical caution, there is an FIGURE 5.1 The Andromeda Galaxy, M31. SOURCE: Image from Robert Gendler. Copyright 2005 Robert Gendler, www. empirical discovery that comes from radio-wavelength robgendlerastropics.com.

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 THE POSSIBILITY OF LIFE ELSEWHERE IN THE UNIVERSE istry is also the same: life on Earth stores its genetic information as deoxyribonucleic acid, DNA, and uses proteins to do most of the business of metabolizing, motility, and other tasks. A molecule closely related to DNA, called ribonucleic acid, or RNA, is used to mediate between the genetic information in DNA and the construction of proteins according to the genetic plans (Figure 5.2). There are certain viruses that store their genetic information in RNA, but to reproduce, this RNA must be converted to DNA within a host cell, and the DNA-protein reproductive machinery of FIGURE 5.3 The phylogenetic tree of life based on compara- tive ssrRNA sequencing. SOURCE: Courtesy of NASA Astrobiol- that cell must be brought to bear. It is possible—though ogy Institute. there is so far no good evidence for this—that there are single-celled organisms on Earth that are unlike the DNA-protein life that we know and that remain unde- different forms of life might be discovered elsewhere tected. Certainly such life would be invisible to DNA in the solar system or beyond. One might imagine that probes. But in the absence of any evidence, it’s difficult it would be convenient to have a general definition of to speculate much further along these lines. So far, the what life is, apart from any particular details of life on life we know on Earth is DNA-protein life. Earth. At least since Aristotle, there have been efforts Life on Earth builds its proteins by stringing to- to define what life is, or to provide lists of its essential gether, like boxcars on a train, different sequences of characteristics. Many definitions have been proposed. amino acids among 20 that are coded for in the genetic Their one common characteristic is that they all fail. sequence of DNA. A small number of other amino For example, there have been metabolic defini- acids are also occasionally used. But from a large list tions, which try to define life as something that takes of possible amino acids that could exist—some 70 dif- in energy, uses it to perform work, and then excretes ferent types have been found, for example, in certain wastes. But fire—which most would not want to call meteorites—life on Earth uses only a small subset. “alive”—also seems to do these things. In fact, the Most impressive is that DNA similarities can be chemical reaction that powers fire is essentially the used to construct a “phylogenetic tree”—a tree of evo- same as the one that we ourselves use. Thermodynamic lutionary relationships—for all known life on Earth definitions claim that life is characterized by a use of (Figure 5.3). These trees make it clear that all known energy to create local order, but mineral crystals do the life on Earth is related and, in fact, can be traced back same, and most scientists would not want them to count to a “last common ancestor.” The exact nature of this crystals as “life.” This is a common problem: proposed last common ancestor is debated, but the relatedness of definitions either include things that do not seem to be Earth life is not. There is only one known form of life alive, or exclude things that we do consider living. Even on Earth, with a common origin. the popular genetic or Darwinian definitions for life Some laboratories are getting close to making forms seem to exclude certain entities that are unambiguously of life (by some definitions of “life”) other than DNA- alive, but that are not capable of Darwinian evolution. protein life, and of course it is possible that altogether The philosopher Carol Cleland and I have ar- gued that this general problem should not surprise us. We have analogized the current situation to that facing Leonardo da Vinci when, five centuries ago, he grappled with what “water” is. There is a page in his Arundel Codex on which he lists the contradic- tory characteristics of water—he considers only liquid water—noting that sometimes it’s yellow, sometimes FIGURE 5.2 Ribonucleic acid (RNA) mediates between deoxy- green, sometimes muddy; sometimes bitter, sometimes ribonucleic acid (DNA) and proteins.

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0 FORGING THE FUTURE OF SPACE SCIENCE sweet, and so on. It’s just very hard for Leonardo to say Since Lederberg’s landmark paper, other words what the fundamental nature of water is. In retrospect, meant to encompass the field have been proposed. this should not surprise us. Leonardo was trying to un- “Cosmobiology”—the biology of the cosmos—is one derstand “water” at a time before there was any theory that particularly appeals to me, but is seldom used. of atoms and molecules. Once such a theory exists, it “Bioastronomy” is also used, but the most prevalent is easy to say what water is—water is H2O—full stop, term now in the United States is “astrobiology,” de- end of story. This clarity comes not from a “definition” fined to mean the study of life in the universe. With of water, but rather a theoretical identity statement. In this definition, there is no artificial—and scientifically the context of molecular theory, water can be precisely unwise—division between the study of life on Earth identified, and there is no ambiguity. Water is H2O, and and the study of possible life elsewhere. that tells us what we mean, even if there are impurities that make a liquid solution sweet, or green, and even ASTROBIOLOGY IN THE SOLAR SYSTEM if the water is frozen as a solid or boiled into a vapor. But this precision is only possible in the context of an The past half-century of solar system exploration has appropriate theory. reinforced the lesson that no arbitrary division should But currently, we have nothing analogous to mo- be placed between life on Earth and astrobiology. Con- lecular theory in our efforts to understand life. We do sider what has been learned about Earth’s Moon. It may not even know if such a general theory of life is possible. be true that the primary drivers for lunar exploration In its absence, it’s hard to see how a definition of life were political rather than scientific, but the scientific will answer any scientific questions for us. Definitions payoff of lunar samples returned to Earth—primar- do not answer scientific questions about the world. On ily by the Apollo missions but also by Soviet robotic the other hand, it may be impossible to devise a general Luna missions—has been huge. Much of what we theory without the perspective that will come from now understand about early solar system history, and discovering other forms of life—should other forms, in therefore early Earth history, begins with the Moon fact, exist, and should we be able to recognize them. missions. This is because the surface of Earth is young, even though Earth is not. Earth is 4.6 billion years old, THE STUDY OF LIFE IN THE UNIVERSE The study of life beyond that we know on Earth was famously given the name “exobiology” in a ground- breaking paper published in Science in 1960, titled “ Exobiology: Approaches to Life Beyond Earth,” written by the Nobel prize-winning biologist Joshua Lederberg. In 1964, another biologist, George Gay- lord Simpson, published something of a reply paper in Science titled “The Nonprevalence of Humanoids,” in which he famously scorned exobiology as a science whose subject matter may not exist. This is rhetori- cally powerful at first glance, but in fact puzzling from the point of view of an astrophysicist: in fact, much cutting-edge work in astrophysics, in physics, and even in fields such as materials science concerns entities or phenomena that may not exist. The Higgs boson, higher dimensions of spacetime, room-temperature superconductors—all could turn out not to exist. It is a strange view of science that this means that their FIGURE 5.4 The Moon. SOURCE: Courtesy of P.-M. Heden of investigation is somehow risible. Vallentuna, Sweden.

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1 THE POSSIBILITY OF LIFE ELSEWHERE IN THE UNIVERSE but there are nearly no rocks left on its surface—due to destruction by plate tectonics and erosion—to tell the tale of early conditions on our own planet. Yet the ancient sedimentary rocks we do have hint that life was established very early on, probably by 3.5 billions years ago, and possibly by 3.8 billion years ago. The Moon, however, died geologically billions of years ago, so preserves much of its record from these early dates. This history, built upon the dating of lunar samples correlated with crater counts on the lunar surface, reveals that the Moon was once subject to an intense bombardment of comets and asteroids—a bombard- ment exponentially higher prior to 3.8 billion years ago than is the case today. Comparison of the lunar cratering record to that of Mercury and ancient Mars suggests that the entire inner solar system was subject to this same bombardment. Therefore the origin of life on Earth must have taken place in the midst of this bombardment, with important implications both for destruction and delivery of carbon-bearing (so-called organic) molecules of use for the origin of life. To learn this about the conditions for early life on Earth, we had FIGURE 5.5 Evidence for recent liquid water on Mars—the to visit the Moon and planets. south-facing walls of Nirgal Vallis. SOURCE: NASA/JPL/Malin Space Science Systems. MGS MOC Release No. MOC2-240. Casting our view farther out from the Sun, the planet Mars is one of the most intriguing possible ven- ues for ancient or even extant life in the solar system. Among such venues, it is also most easily accessible lies the moon Europa, just a bit smaller in size than from Earth, with spacecraft travel times that are less Earth’s Moon. There is now strong evidence that than one year. Spacecraft flybys, orbiters, landers and Europa harbors an ocean of liquid water beneath its rovers have made it clear than ancient Mars once had extremely cold outermost layer of ice (Figure 5.6). The abundant liquid water at its surface, and there is strong volume of this ocean is about twice that of Earth’s evidence that, in specific locations at specific times to- oceans. At the floor of Europa’s ocean, as on Earth, liq- day or in the geologically very recent past, liquid water uid water is in contact with rock, raising the possibility still reaches and flows at the surface (Figure 5.5). The of important water-mineral interactions in the presence surface itself is now a freeze-dried desert where liquid of hydrothermal energy. Data from the magnetometer water must either freeze or evaporate. But given what on the Galileo spacecraft not only supports the existence we’ve learned about the deep biosphere on Earth, the of the ocean, but suggests that it is very salty and that possibility that life on Mars exists in subsurface liquid the overlying ice may be only 10 kilometers thick, or water environments—environments that may occasion- even thinner. Could there be life in this ocean? Specula- ally reach the surface—must be taken seriously. Because tive studies suggest that the energy sources needed to of their proximity, Mars and Earth may exchange mete- support life should be present. But whether the origin orites that are created as ejecta from large impacts, and of life could have occurred in an ocean that was beneath it is not out of the question that whichever planet first kilometers of ice—so likely cutoff from sunlight—is an originated life could than have inoculated the other. open question. It is much harder for Earth and Europa Only discovering and examining possible martian life to successfully exchange microorganisms via meteorites could answer this question with certainty. than is the case for Earth and Mars, so if there is life on Beyond Mars, in orbit around the planet Jupiter, Europa, it is likely due to a separate origin from life on

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2 FORGING THE FUTURE OF SPACE SCIENCE FIGURE 5.6 Cross-sectional diagram of Europa’s 80–150 km thick H2O layer assuming a metal core surrounded by rock mantle: An intermediate subsurface slush also remains a possibility. SOURCE: Courtesy of NASA/JPL. Available at http://photojournal.jpl.nasa. gov/catalog/PIA01669. Earth. But because of the liquid water ocean, Europa question. The Outer Space Treaty, which entered into may be the most intriguing site for extraterrestrial life force in 1967 and is best known for forbidding the in our solar system. It appears that Jupiter’s Mercury- placement of “weapons of mass destruction” in outer sized moons, Ganymede and Callisto, harbor deeper space, requires space-faring nations to avoid the “harm- subsurface liquid water oceans as well. ful contamination” of other celestial bodies. Within a Still farther out from the Sun, the planet Saturn decade, then, Lederberg’s personal concern had given hosts at least two intriguing worlds. The Cassini space- way to an international treaty requirement. craft has revealed that tiny Enceladus has active geysers The concern is scientifically well founded. In- of ice crystals that may originate in a subsurface sea vestigations with NASA’s Long-Duration Exposure of liquid water, though the exact mechanism for the Facility (LDEF) and European Retrievable Carrier geysers and whether there is enough energy to sustain (EURECA) experiments reveal that certain micro- liquid water in Enceladus’ subsurface remains to be organisms survive 6 years in space at the 1 percent convincingly argued. Farther out from Saturn lies the level—i.e., one out of a hundred Bacillus subtilis spores Mercury-sized world Titan, with its dense atmosphere survive for this long—whereas 25 percent survive a of nitrogen and methane. There is some evidence that year in space. In both cases, survival requires that the Titan, too, may harbor a subsurface liquid water ocean. organisms are shielded from the Sun’s ultraviolet light, All of these worlds need much more exploration and but any organism inside a spacecraft would be. The should receive it later this century. Missions to the outer organisms freeze-dry, or lyophilize, in the cold vacuum, solar system take time (the travel time to Jupiter is 3 but when introduced to liquid water they revive. Most years from Earth) and are expensive. But a balanced NASA Mars mission spacecraft are constructed in program of solar system exploration, especially one c lass-100,000 clean rooms, which means they have emphasizing astrobiology, must systematically explore thousands of viable sporulating bacteria present per the Jovian and Saturnian systems as well as Mars. square meter of spacecraft surface, and probably ten or more times as many other types of bacteria. Since it takes less than a year to get to Mars, this means that PLANETARY PROTECTION Mars spacecraft carry a viable bioload of microorgan- An important issue in planetary exploration is planetary isms with them to the Red Planet. The first question protection. It was Lederberg who, during the IGY in becomes, then, whether any of these organisms could 1957, wrote to the president of the National Academy find their way from the martian surface into habitable of Sciences to raise this as an issue, and the Academy niches with liquid water in the subsurface, and if so, worked with the International Council of Scientific whether they could grow in that new environment. Unions to create an international study group on this The odds are long, but not impossible. The second

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 THE POSSIBILITY OF LIFE ELSEWHERE IN THE UNIVERSE FIGURE 5.7 An overall side view of the Long Dura- tion Exposure Facility grappled by remote manipulator system during STS-32 retrieval. SOURCE: NASA Lang- ley Research Center. Image # EL-1994-00078. question is what level of additional measures to reduce FOUR WAYS TO LOOK FOR LIFE the bioload of Mars spacecraft should be taken during So far we have discussed in situ investigation of the spacecraft construction. A recent report that I chaired solar system, in which spacecraft land on other bod- for the National Research Council (NRC), P reenting ies and conduct experiments at the surface to look for the Forward Contamination of Mars, looked at these life. Closely related is the biological examination, in questions and concluded that NASA needs to better terrestrial laboratories, of samples from other worlds. understand the numbers and types of microorganisms These samples could arrive on Earth in an uncontrolled that currently fly on its spacecraft and take more strin- way, via meteorites that originated as debris blown off gent steps to reduce spacecraft bioload. another world by a big impact, or in a controlled way, The current international interpretation of the as samples brought back by a dedicated spacecraft. But Outer Space Treaty requirement is that microorgan- both cases involve hands-on investigations for the pres- isms carried to other planets must not be allowed to ence of life in the solar system. take hold on that world in a way that would render it A third way to search for life is to examine the light difficult or impossible to determine if a truly alien bio- coming from the atmospheres of other worlds—i.e., sphere might be present. That is, planetary protection spectroscopy—to determine the chemical composition as it stands is really about “protecting the science” from of those worlds’ atmospheres in the hope of finding the contamination, not about protecting any possible alien chemical signatures of another biosphere. This has been biosphere from potential ecological assault. Our NRC done for Mars and other planets in our solar system for report urged that it was time for an international meet- decades and has just become possible for certain giant ing to reconsider whether planetary protection should exoplanets—planets in orbit around a star other than be reinterpreted to be about “protecting the planet,” and our Sun. not just “protecting the science.”

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 FORGING THE FUTURE OF SPACE SCIENCE FIGURE 5.9 NASA’s first mission capable of finding Earth-size and smaller planets. SOURCE: Courtesy of NASA. With the Kepler mission that will launch in the stantial, and as soon as such data were reported, sci- next few years, we should soon know the statistics of entists would rightly, and conservatively, search for the presence of Earth-sized planets around other stars. non-biological explanations. Indeed, we have seen [Figure 5.9] Kepler will allow us to determine the or- this already at Mars: it is now clear that the martian bits of these planets (assuming that there are any) and atmosphere, a highly oxidizing atmosphere flooded therefore their distances from their stars. With knowl- with ultraviolet light that should not permit organics edge of the stars, we will know which, if any, of these to exist for long, contains patches of the simple organic worlds lie in the right range of distances for liquid water molecule methane—at about the 10 parts per billion oceans to be possible on their surfaces. In a few years’ level. The methane must be produced by localized time, we will go from almost no knowledge of whether sources at the surface; it is well out of equilibrium with other Earth-like planets exist to knowing their statistics the existing atmosphere. It could be the product of a and potential surface habitability. This is an extraordi- martian version of the methanogenic bacteria we know nary moment. Humans have speculated for millennia on Earth. But already there have also been published about whether other planets like ours could exist—for papers suggesting explanations in terms of martian example, Aristotle asked (and answered, on theoretical geochemistry. Atmospheric chemistry consistent with grounds) this question in his book On the Heaens. In biological sources may provide hints of life, but it evi- a few years we will no longer have to speculate. We dently it does not in itself provide decisive arguments should not let human civilization sleepwalk through for the existence of life. this remarkable transition in our knowledge about our place in the universe. SETI1 Some decades further on we will be able to observe these planets from dedicated satellites in space, and Besides the three techniques for searching for extra- determine the composition of their atmospheres. The terrestrial life so far discussed—in situ investigations, hope is that we might detect some combination of examination of samples delivered to Earth, and remote gases in some atmospheres that equilibrium chemistry sensing of planetary atmospheres—there is one other would seem to forbid, but which biology might just 1*This and the following three sections draw on a more techni- generate. This could imply that there are biospheres cal discussion in Christopher F. Chyba and Kevin P. Hand, “As- on these worlds. trobiology: The Study of the Living Universe,” Annual Reiew of Or maybe not. The evidence would be circum- Astronomy and Astrophysics, vol. 43 (2005), pp. 31-74.

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 THE POSSIBILITY OF LIFE ELSEWHERE IN THE UNIVERSE approach to the search for life that human civilization up against multiple tests, including a check against all currently has underway. This is the search for extrater- known confounding signals (e.g., from Earth-orbiting restrial intelligence (or, rather, technology), or SETI. satellites or interplanetary probes), a requirement that SETI need make no assumptions about the biochemi- the frequency be so well defined (i.e., that the band- cal or other makeup of extraterrestrial life. It must, on width be narrow) as to only be possible artificially, and the other hand, rely on the existence of technology ca- a demonstration that the source was detected not only pable of communicating across interstellar distances. at Arecibo but on a follow-up radiotelescope in Britain The most powerful targeted search to date has as well. No source has ever passed through all of these been the SETI Institute’s Project Phoenix, which filters. observed roughly the thousand nearest Sun-like stars It is sometimes said that humanity has looked and for radio-frequency broadcasts. Phoenix completed its looked and looked for extraterrestrial radio transmis- search at the Arecibo radiotelescope in Puerto Rico, sions without finding any, so it must be that we are the world’s largest, and therefore most sensitive, radio alone. Superficially, this might seem to follow from receiver (Figure 5.10). Radio frequencies are the natu- the fact that SETI radio searches have been carried out ral frequency to use for interstellar communications, since the first search was conducted by Frank Drake because of the so-called microwave window where nearly 50 years ago. But in fact, even Project Phoenix galactic background noise is lowest. For each target has only scratched the surface. The nearly 1,000 stars it star, Project Phoenix examined billions of frequencies. has searched account for just a ten-millionth of the stars Algorithms assumed that the frequency would drift, as in our galaxy. The SETI Institute and the University a real transmission certainly would due to the motion of California are now constructing the Allen Telescope of the source of the transmission relative to Earth. To Array (ATA) in northern California using almost be a credible detection, any signal received had to stand entirely private funds (Figure 5.11). This array will F IGURE 5.10 T he 1,000- foot (305-meter) dish at Are- cibo, Puerto Rico, is the most sensitive radio telescope in the world. It was used by Projects Phoenix and SERENDIP, and it’s currently feeding huge vol- umes of data to SETI@home. SOURCE: NAIC Arecibo Ob- servatory, a facility of the Na- tional Science Foundation.

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 FORGING THE FUTURE OF SPACE SCIENCE FIGURE 5.11 Artist rendering of completed ATA-350. SOURCE: Courtesy of Isaac Gary. perform SETI searches all day every day (rather than a hundred million other Earth-like worlds, but only one the few weeks per year that were possible at Arecibo), would have hit the jackpot. This would be like rolling using the most recent technology. Once completed, six identical dice and having only one came up with a the ATA should examine around a million stars in a six. There is nothing special about that particular die; decade of observing. But even this will represent just any one of them could have rolled a six but statistically a hundred-thousandth of the stars in our galaxy. If most of them would not. The Copernican principle is technical civilizations beaming signals across interstel- not violated, but Earth could still be unique. lar distances are more rare than one in every hundred The Drake equation summarizes this way of looking thousand stars, even the ATA will not be successful at the problem. Frank Drake wrote down his equation anytime soon. But in the absence of any mature theory as a meeting agenda for a workshop on SETI in 1961. about the prevalence of intelligent life and technology, The Drake equation reads: N=R*fpneflfifcL, where N is the search is the best that we can do. the number of technically communicative civilizations Nevertheless, arguments have been put forward in our galaxy, R* is the galaxy’s rate of star formation, fp regarding the likelihood of extraterrestrial intelligence. is the fraction of those stars around which planets form, Perhaps the most common and intuitive is the simple ne is the number of planets in such systems suitable comment that with so many stars—hundreds of billions for the origin of life, fl is the fraction of those planets in our galaxy alone—it just can not be that we’re the on which life originates, fi is the fraction of those on only civilization. On the face of it, this assertion also which life evolves intelligence, fc is the fraction of those seems consistent with the Copernican principle, the intelligent species that become communicative across idea that Earth has no unique status in the universe. interstellar distances, and L is the average lifetime of a But in fact, this line of reasoning does not hold up. The communicative civilization. reason is that we do not know the probability of the Obviously this equation is not an equation analo- origin of life, and then intelligence, and then technol- gous to, say, the ideal gas law equation. The ideal gas ogy, on an Earth-like world. If this probability were law hypothesizes a relationship among the pressure, extremely small—say, less than one in a hundred bil- volume, and temperature of gases in the laboratory, so lion—then Earth could be the only planet in the galaxy is subject to empirical test. The Drake equation does harboring an intelligent civilization. There could still be not pose this kind of testable hypothesis. Rather it is

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 THE POSSIBILITY OF LIFE ELSEWHERE IN THE UNIVERSE a type of “Fermi problem,” an example of the sort of replay” as Stephen J. Gould wrote in 1989. The evolu- back-of-the-envelope thinking made famous by Enrico tion of human intelligence, after all, depended on a Fermi in his graduate examinations, by asking questions series of contingent factors, including the collision of like “How many piano tuners are there in the city of a major asteroid with Earth 65 million years ago. The Chicago?” At first glance, you either know the answer counterarguments are equally familiar: convergence is to this question or you do not, and if you do not, there frequently observed in evolutionary history, and nature is no easy way to figure it out. But in fact, by breaking has evolved complex phenomena such as eyesight and the calculation down into a product of numbers that flight many times, so that even though any given evolu- may be estimated (such as the population of Chicago, tionary line might be highly contingent, a large number the number of people per family, the fraction of families of parallel paths may lead to the same functional out- that own pianos, how often pianos have to be tuned, come. To this the reply is made that technical intel- and so on) one can make a reasonable estimate of the ligence has only evolved once on Earth, so evidently correct answer. convergence was not operating in this particular case. But this can not be done with the Drake equation. But things are not this clear-cut; as the marine biologist W hile the three factors R*, fp, and ne can be assigned Lori Marino’s work has emphasized, several species of credible estimates on the basis of what we already marine mammals developed a level of intelligence that know, the remaining factors can only be guessed. L, in by quantifiable measures is in excess of that of chim- particular, moves us into the realm of extraterrestrial panzees and slightly in excess of that of homo habilis, sociology and political science, which remain less devel- one of modern humans’ tool-using ancestors. oped fields. At its upper end we might imagine that L Marino and her colleagues begin with a reproduc- could be as long as the age of the galaxy, ~1010 years. At ible measurement that correlates with what is meant its lower end it could be as short as the interval between, by intelligence, and that can be employed with the say, the invention of radio and the mass production fossil record as well as contemporary organisms. There of thermonuclear weapons; based on our experience, is at least one such measure, called encephalization. this interval could be as short as decades. The aver- Encephalization is typically expressed as a quotient age value of L in the galaxy might well be anywhere (hence, encephalization quotient, or EQ) that quanti- in this interval, although even a small number of very fies how much smaller or larger a particular animal’s long-lived civilizations could make the average quite brain is compared to the expected (via a regression over long indeed. In the face of the uncertainties the Drake many animals) brain size for an animal of that body equation reveals, the large-numbers argument cannot size. Animals with EQs above 1 are brainier than aver- resolve questions about the frequency of civilizations age; those with EQ values below 1 are less brainy than in our galaxy. expected for their body size. There is strong evidence that EQ among primates correlates with the ability to innovate, social learning and tool use; among birds it INTELLIGENCE ON EARTH correlates with behavioral flexibility. It seems, therefore, Another way to assess the prospects for other intelligent to provide a good measurable proxy for “intelligence.” life is to extrapolate from the history of life on Earth. Contemporary humans have the highest EQ on Earth There is a set of arguments bearing on this question at 7.1, meaning that our EQ is more than 7 times greater that have been rehearsed for a full century, beginning than expected for an animal of our body weight. in 1904 with Alfred Russel Wallace, the co-discoverer In well-controlled studies, dolphins have been of the theory of evolution, and being revived at inter- shown to be capable of mirror self-recognition, an abil- vals since by a series of authors. The pessimists in this ity demonstrated only by a few other animals besides argument emphasize the contingency of evolution, for humans (Figure 5.12). The highest EQ values on Earth example how if one were to replay the evolution of after modern humans are those of four dolphin species, animals, the results would likely be very different, and with the highest of the four being about 4.5. Great apes in particular “the chance becomes vanishingly small have EQs lower than this, with a mean around 1.9. that anything like human intelligence would grace the This is about the same as that of the human ancestor

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 FORGING THE FUTURE OF SPACE SCIENCE * = p < 0. 05 ns EQ * ns * * * * Pacific White ided Bottlenose dolphin Common dolphin -s dolphin s c tu re .e H 3.1 2.8 2.5 1.9 1.8 .9 .18 .05 .02 0 Mean Geological Age FIGURE 5.12 SOURCE: Courtesy of L. Marino. (millions of yrs) Australopithecus. Among our more recent ancestors, the verse. Just as studies of microscopic life on Earth in- tool-users Homo erectus and the earlier Homo habilis had form thinking about the prospects for microorganisms EQ values of about 5.3 and 4.3, respectively. elsewhere, so can rigorous exploration of the evolution These results suggest that the evolution of human of intelligence on Earth inform our thinking about the intelligence on Earth is not an entirely exceptional prospects for intelligence elsewhere. Treating “intelli- phenomenon. With a sufficiently large database of gence” as a property of the biological universe that can EQ measurements for fossil whale species, one can go be quantitatively investigated should allow us to move further, and begin to test other long-standing assertions beyond polemics and begin to push back the boundaries about intelligence, such as the claim that increases in of our ignorance in a data-driven fashion. encephalization should be pervasive because of the selective advantage that is conferred by bigger brains. ASTROBIOLOGY AND THE Marino and her colleagues have done this analysis, HUMAN FUTURE applying statistical tests to data for modern and fossil whales going back 50 million years. They show that Fermi posed his famous question “Don’t you ever while the overall trend in encephalization has been wonder where everybody is?” to three colleagues at Los increasing, at any given speciation event, the successor Alamos National Laboratory in 1950. In its modern species was as statistically likely to have a lower EQ as version, the “Fermi paradox” maintains that if other a higher one. That is, encephalization was not perva- civilizations exist in the Milky Way galaxy, some must sively advantageous; the increase in intelligence at the be much older, perhaps billions of years older than high end of encephalization seems better modeled as a ours; that such civilizations would long ago have de- random walk rather than a pervasive selection pressure veloped interstellar travel; that they would then have favoring bigger brains. But it should be emphasized explored or colonized the galaxy on a timescale that that the size of the data set here is so far very small, and is short compared with the galaxy’s lifetime; and that nearly no funding is available for this kind of work. they would therefore be here. But since they are not These results are those of only a nascent research here, they must not exist! The paradox obviously does program, but they emphasize that there are reproduc- not hold in a strict logical sense, since each of its as- ible, quantitative methods that can be applied to begin sertions is at best a claim of probability, but it has been to address some long-standing assertions about the a powerful force on thinking about the prospects for likelihood of the evolution of intelligence in the uni- extraterrestrial intelligence.

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 THE POSSIBILITY OF LIFE ELSEWHERE IN THE UNIVERSE Whatever the rigor of the Fermi paradox, there is exponentially advancing at a rate even faster than have been many solutions proposed for it. The chal- that of Moore’s law in computing. It is hard to know lenge to most of these solutions is the large-number what comes after this exponential lift-off. It may prove assertion: while this or that explanation might explain generally true that there is only a brief interval dur- the failure of some, even most, civilizations to colonize ing which a species is technically intelligent yet still the galaxy, the timescale for colonization is putatively retains its biologically evolved form. If so, we should so short that unless the total number of civilizations in expect that any civilization with which we make contact galactic history were quite small, the galaxy would in- through SETI or otherwise is unlikely to resemble its deed have been colonized. These colonization scenarios biological predecessor species. If the question is “what have posited exponential reproduction and paid little will they look like?” the answer may be “whatever they attention to ecological factors, such as the evolution want to.” of predation or other behavior that could have the ef- But well before biotechnology permits the reengi- fect of reducing the rate of expansion of a space-faring neering of the human species, it will put great power population. What parameters does one choose in for extremely dangerous manipulations of microorgan- predator-prey modeling to depict accurately the expan- isms into the hands of small groups of the technically sion timescales of competing technical civilizations? It competent. Indeed, it is doing so already. (The National is hard to make such parameter choices with a feeling Academies has already convened two committees to of confidence. And it is close to impossible to know examine this issue.) We do not have adequate models whether such simple analogies from life on Earth are from Cold War arms control or nuclear nonprolifera- or are not applicable. tion for how to manage this new world, gaining the Various practical arguments against galactic space- benefits of biotechnology for public health and food flight being commonplace have been countered by security while preventing disaster. The same techno- invoking either genetic engineering or artificial intel- logical expertise that makes possible our increasingly ligence in the form of self-replicating and evolving ma- sophisticated searches for life brings with it powerful chines. We should not exaggerate the ease or casualness new opportunities, if mishandled, for destruction. with which substantial genetic manipulation of human Astrobiology is defined as “the study of the living uni- beings will be done, but as Robert Carlson has shown, verse.” If so, then the discipline must also speak to the basic measures of human bioengineering power, such future of human civilization, a thing uniquely precious as the time or cost required to sequence or synthesize regardless of whether it is entirely alone or one of many short sequences of DNA, show that biotechnology in the galaxy.